This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following conventional notations are used in this description:
Bold uppercase and lowercase denote the matrices and vectors respectively,
(•)T: the transpose operation of the matrix.
(•)*: the conjugate operation of the matrix or the element.
(•)H: the conjugate-transpose operation of the matrix.
tr{•}: the trace of the matrix.
diag{•}: the diagonal matrix formed by the given elements
[y]+: the maximal value of the y and zero.
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-long term evolution LTE or as E-UTRA) is currently under development within the 3rd generation partnership project (3GPP). LTE-A is an advancement to the basic (Release-8) LTE system in which relay stations are more prominent. The IEEE 802.16m also makes more significant use of relay stations. Each of these systems are exemplary but non-limiting environments in which these teachings may be employed.
There has been significant research into relay systems in recent years. With a relay station (RS) between a source station (SS) and a destination station (DS), the SS does not need to use a high transmit power to extend its coverage. There are considered two primary approaches for relay systems: an amplify-and-forward (AF) mode; and a decode-and-forward (DF) mode. In an AF, the RS amplifies the signal it receives from the SS and retransmits the amplified signal to the DS. In a DF, the RS decodes the signal received from the SS, recodes it, and transmits the re-coded signal to the DS. These teachings are directed to the AF approach. It is noted that the SS need not be the originator of the signal and the DS need not to be the end user; either or both the SS and the DS may also be RSs. The terms SS and DS are used herein simply to distinguish the direction that the signal moves through the RS.
The design of the gain factor/matrix at the RS is important in an AF type relay system. Different designs can obtain different performance. Some gain matrices are designed to maximize the instantaneous throughput of the system, and some gain matrices are designed to diagonalize the overall MIMO channel at the RS, which will allow the RS to optimally choose the gain coefficient for each eigenmode while fulfilling the transmit power constraint. An example of the former may be seen in a paper by Ingmar Hammerstrom, Marc Kuhn, and Armin Wittneben entitled: “IMPACT OF RELAY GAIN ALLOCATION ON THE PERFORMANCE OF COOPERATIVE DIVERSITY NETWORKS” (IEEE 60th Vehicular Technology Conference, VTC2004-Fall: Wireless Technologies for Global Security, 2004, p 1815-1819). An example of the latter may be seen in a paper by Markus Herdin, entitled: “MIMO AMPLIFY-AND-FORWARD RELAYING IN CORRELATED MIMO CHANNELS” (2005 Fifth International Conference on Information, Communications and Signal Processing, 2005, p 796-800).
No matter which criterion is satisfied, all of the gain matrices perform amplification to the signal received by the RS, for all of them are just multiplied by the signal and the operation is linear.
The gain factor/matrix is designed following some criterion in downlink and uplink. The channel information of the first hop is usually used as a part of the gain factor in the design. See for example a paper by Xiaojun Tang and Yingbo Hua, entitled: “OPTIMAL DESIGN OF NON-REGENERATIVE MIMO WIRELESS RELAYS” (IEEE Transactions on Wireless Communications, v 6, n 4, April, 2007, p 1398-1406). But the channel of the first hop in the downlink and the uplink are totally different, which will lead the gain matrices of the downlink and the uplink to be different although they may be designed following the same criterion.
With the above approach in the frequency division duplex (FDD) mode, when the SS needs to know the whole link channel information before the signal was sent, the SS can simply receive feedback from the RS or DS prior to sending that signal. This of course incurs an extra expense of signaling overhead, and introduces much delay in the whole relay procedure. Therefore the actual link performance is degraded, and this type of degradation is more serious than the system without a multi-hop relay network. Further details of this conclusion can be seen in a paper by K. Byung, Yi, Shu Wang Yi. Y. Soon, and Kwon, entitled: “ON MIMO RELAY WITH FINITE-RATE FEEDBACK AND IMPERFECT CHANNEL ESTIMATION” (Global Telecommunications Conference, 2007. GLOBECOM '07. IEEE 26-30 Nov. 2007 Page(s):3878-3882).
What is needed in the art is a more efficient approach to AF relaying.